1. Field of the Invention
This invention pertains to low NO.sub.x combustion turbine combustors. More particularly, the pilot flame, used to heat and ignite the fuel and air mixture within a turbine combustor, is implemented as an electric arc plasma torch. This enables heating and ignition of fuel within the turbine combustor without the associated need for combustion air introduced by a conventional pilot flame.
2. Prior Art
Various industries such as coal-fired electrical power generation and oil and gas production produce nitrogen oxide gases (e.g. NO, N.sub.2 O, NO.sub.2, and N.sub.2 O.sub.4), the key factor being combustion of fossil fuels. Nitrogen oxides (collectively referred to as NO.sub.x) are primary air pollutants and, as such, provoke considerable interest in the development of efficient, cost-effective technologies to remediate NO.sub.x -containing emissions. N.sub.2 O is thought to be a major contributor to global warming due to its persistence in the atmosphere (about 150 years) and its relatively high infrared absorbance (&gt;200 times that of carbon dioxide). William A. Apel & Charles E. Turick, The Use of Denitrifying Bacteria for the Removal of Nitrogen Oxides from Combustion Gases, 72 Fuel 1715 (1993). In view of the foregoing, it will be appreciated that a process of removing nitrogen oxides from processes that normally produce them would be of great benefit.
Turbine combustors are presently producers of nitrous oxide gases because they burn fossil fuels. The simplest combustion control technology for reducing NO.sub.x production is low-excess-air operation. A conventional combustor uses a pilot flame within the combustion turbine combustor basket to heat and thereby ignite the fuel and air mixture. The fuel and air mixture introduced into the combustor basket is kept lean so as to reach an optimal fuel/air ratio to minimize NO.sub.x produced by the turbine. Nevertheless, the pilot flame used to ignite the fuel-air mixture is a relatively fuel-rich flame. Consequently, the turbine produces NO.sub.x because the high temperatures within the rich pilot flame combine to form NO.sub.x from the excess nitrogen and oxygen in the pilot flame combustion air. Therefore, the pilot flame is the greatest source of NO.sub.x produced by combustion turbines.
It is apparent that while using a lean fuel mixture is an effective method of controlling NO.sub.x production within a turbine combustor, the benefits of this arrangement are substantially counterbalanced by the fuel-rich pilot flame used to ignite the lean fuel/air mixture. It would therefore be an advantage over the prior art if the combustion air of the pilot flame could be reduced in order to reduce overall NO.sub.x production from the combustion turbine combustor basket. It would also be an advantage if the pilot flame could be used to actually reduce NO.sub.x production by converting NO.sub.x back to N.sub.2.
Another problem of the low-excess-air approach to reducing NO.sub.x emissions is that the combustor is subject to an instability in combustion of the fuel and air. This is mainly caused by the fact that if the fuel and air mixture is not mixed well, combustion within the combustor basket becomes erratic. This erratic state is manifested when the lean fuel/air mixture stops combusting momentarily until a sufficient concentration of fuel and air builds up again within the combustor basket and ignites. This flickering combustion effect within the combustor basket can become so violent that the turbine can tear itself apart from the constant reignition of fuel and air. Unfortunately, conventional chemical pilot flames are not able to adjust rapidly enough to changing conditions within the combustor basket. Therefore, it would be another advantage of the present invention if a pilot flame could be developed which could adjust rapidly enough to compensate for changing combustor conditions and fuel/air mixture ratios sufficient to stabilize combustion within the combustor basket.